(1) Field of the Invention
This invention relates to a porous membrane permitting permeation of a hydrophilic liquid therethrough and a production process thereof.
(2) Description of the Prior Art
The fields of application of porous polyolefin membranes is growing rapidly due to their excellent mechanical properties and chemical resistance. Porous polyolefin membranes are however hydrophobic, and when used as is water permeates therethrough with difficulty. A hydrophilizing treatment is therefore indispensable to have hydrophilic liquids including water to permeate therethrough. A variety of methods have been studied with a view toward imparting hydrophilicity through surface modification of polyolefin membranes. Hydrophilizing methods, which have been proposed for film-like materials featuring smooth surfaces, cannot be simply applied to impart hydrophilicity to porous membranes having complex surface configurations.
As hydrophilizing methods for porous polyolefin membranes, there have been known the organic solvent wetting and water substituting method, the physical adsorption method, the chemical surface modification method and other methods. In the organic solvent wetting and water substituting method, the entire surface of a porous polyolefin membrane, inclusive of minute pores, is subjected to a wetting treatment with a organic solvent having good miscibility with water such as an alcohol or ketone, followed by the substitution of water for the organic solvent. If water is once lost from minute pores during storage or use, the part containing these water-free minute pores regains hydrophobicity and no longer permits the permeation of water therethrough. Accordingly, it is always necessary to keep water around the porous membrane in the above method. The porous membrane is therefore cumbersome to handle. According to the physical adsorption method, a hydrophilic material such as polyethylene glycol or a surfactant is adsorbed on the pore walls of a porous membrane so as to impart hydrophilicity to the porous membrane (see, for example, Japanese Patent Laid-Open Nos. 153872/1979 and 24732/1984). Although this method is easy to practice, the hydrophilic material drops off if the resulting porous membrane is used over a long period of time. Therefore, this method cannot be regarded as a satisfactory hydrophilizing method.
As a chemical surface modification method, it has been proposed, for example, to expose a porous membrane to radiation while holding a hydrophilic monomer on the surface of the membrane (Japanese Patent Laid-Open No. 38333/1981) or to subject the porous structure of a hydrophobic resin to a plasma treatment in a state impregnated with a water-soluble high-molecular material and a surfactant (Japanese Patent Laid-Open No. 157437/1981). These methods are however accompanied by one or more problems. It is difficult to impart uniform hydrophilicity in the direction of the thickness of a membrane, no matter which of these methods is relied upon. If one attempts to apply a hydrophilizing treatment uniformly over the entire thickness of a porous membrane when the membrane has a large thickness or is in the form of hollow fiber, the mechanical strength of the matrix of the porous membrane is unavoidably reduced.
As has been described above, no effective hydrophilizing method has been established to date for porous polyolefin membranes.